In semiconductor fabrication, circuits are formed on wafers of a semiconductor material such as silicon. A single crystal of the semiconductor material is sliced into thin wafers and the wafers are transported between various stations, such as processing stations, storage stations, or queuing stations, in the fabrication plant. The fabrication plant must be kept clean to prevent contamination of the semiconductor wafers. The wafers are accordingly handled very carefully in sealed cleanroom environments.
Robots are often used to transport the wafers between processing, storage, queuing or other stations. A typical robot includes an arm movable vertically and in a horizontal plane. An end effector on the end of the arm picks up and places the wafer. The end effector may include a mechanism for holding onto the wafer to prevent the wafer from slipping or otherwise becoming damaged while the robot arm is carrying it.
In certain semiconductor applications, the wafer includes an alignment feature, such as a notch or flat, along one edge that is used to align the wafer in a desired orientation prior to delivering the wafer to a processing station. A pre-aligner device is often used to accomplish this alignment. Before delivering the wafer to the processing station, the robot arm deposits the wafer at the pre-aligner device, which rotates the wafer until the notch or flat is properly oriented. The robot arm then picks the wafer up again, in the proper orientation, and delivers the wafer to the processing station.